Scalable and modular heat sink-heat pipe cooling system

ABSTRACT

Modular heat sinks utilizing heat pipes to provide a more uniform temperature distribution over a packaged integrated circuit and efficient heat sinking in either free or forced convection environments. The heat sinks utilize both horizontal and vertical heat pipes to transfer heat both horizontally and vertically in the heat sinks. Selection of the number of heat pipes used allows tailoring of the heat sink capabilities for different applications using the same fundamental assemblage of parts. Various embodiments are disclosed.

This appln is a continuation of Ser. No. 08/885,122 filed Jun. 30, 1997,abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of heat sinks forsemiconductor devices and integrated circuits.

2. Prior Art

Various configurations of heat sinks for semiconductor devices andintegrated circuits are well known in the prior art. One common type ofheat sink for mounting on the integrated circuit or other semiconductordevice comprises a heat sink having projecting fins or legs to increasethe surface area of the heat sink for heat dissipation to thesurrounding area, either through free convection or forced convection(fan driven) cooling.

In some cases, heat sinks have been configured to somehow snap onto theintegrated circuit or other semiconductor device. In such circumstances,the thermal contact between the integrated circuit and the heat sink canbe substantially limited, as the actual area of contact between theintegrated circuit and the heat sink can be only a small fraction of apotential area for such contact. In such cases, the heat transfer fromthe integrated circuit to the heat sink may be increased through the useof a thermally conductive grease spanning the air spaces between theheat sink and the packaged integrated circuit. In other cases, the heatsinks have been cemented to the packaged integrated circuits, providingboth the mounting and the substantial absence of air spaces between thepackaged integrated circuits and the heat sinks.

In the foregoing type of heat sinks, even with very good thermalcoupling between the integrated circuit and the heat sink, therefrequently is a substantial differential temperature between theintegrated circuit and the cooling fins or protrusions on the heat sink,particularly in the larger integrated circuits having a high powerdissipation per unit area. Further, integrated circuits having a highpower dissipation per unit area tend to run hotter in the center of theintegrated circuit than at the edges of the integrated circuit becauseof the lateral flow of heat away from the edges of the silicon chip.This temperature difference across the chip is undesirable, as evenidentical transistors operating at different temperatures have differentcharacteristics. The temperature differentials across the chip alsomechanically stress the chip, and also allow part of the chip to runhotter than it otherwise would if the temperature was more evenlydistributed.

Heat pipes are also well known devices for transferring heat from awarmer location to a cooler location. A typical heat pipe is comprisedof an appropriately shaped heat conductive enclosure which has beenpartially filled with an appropriate liquid. In operation, the liquid inthe portion of the heat pipe adjacent the hotter area of the heat pipeabsorbs heat and turns to gas, with the gas adjacent the cooler area ofthe heat pipe condensing back to liquid form to flow back to the hotterarea of the heat pipe. Thus, a flow of gas is established from thehotter portion of the heat pipe to the cooler portion of the heat pipe,and a corresponding flow of liquid is established back from the coolerportion of the heat pipe to the hotter portion of the heat pipe. Thus,the heat transfer achieved through the use of the heat pipe is primarilya result of the mass transfer occurring within the heat pipeautomatically as a result of the differential temperature between theends of the heat pipe.

SUMMARY OF THE INVENTION

Modular heat sinks utilizing heat pipes to provide a more uniformtemperature distribution over a packaged integrated circuit andefficient heat sinking in either free or forced convection environments.The heat sinks utilize both horizontal and vertical heat pipes totransfer heat both horizontally and vertically in the heat sinks.Selection of the number of heat pipes used allows tailoring of the heatsink capabilities for different applications using the same fundamentalassemblage of parts.

One of the important enhancements of this invention is the ability toprovide uniform and even heat flux (power density) distribution. Thisparticular functionality reduces high junction temperature in asemiconductor (or any power) device (ASICs, microprocessors, power andlaser devices, etc.). This functionality will also enhance and reducethe cooling requirements for these devices because of the heat flux perunit area (i.e., power density) reduction.

The volumetric and surface area heat flux distribution is determined bythe number and spacing of these heat pipes in both lateral and axialdirections, and the available cooling medium. In most cases, for lowpower devices, the application of this invention will be sufficient tocool devices by natural convection.

Various embodiments and fabrication techniques are disclosed, includingboth heat sinks having directional and non directional convectioncurrent dependency.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is an exploded, perspective view of a first embodiment of thepresent invention.

FIGS. 2, 3 and 4 are side, end and top views of the assembly of FIG. 1,respectively.

FIG. 5 shows a heat sink in accordance with the present inventioncemented to the top surface of a packaged semiconductor device such as aball grid array (BGA) package.

FIG. 6 illustrates a heat sink in accordance with the present inventionwhich is air flow direction dependent.

FIG. 7 illustrates a still further embodiment of the present inventionwhich, like that of FIG. 6, is also air flow direction dependent.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

First referring to FIG. 1, an exploded, perspective view of a firstembodiment of the present invention may be seen. The heat sink showntherein is comprised of a substantially planar base member 20 and aplurality of finned, tower-like members, generally indicated by thenumeral 22, extending upward therefrom. The base 20 is a plurality oflongitudinal openings 24 running lengthwise through the base memberwithin which heat pipes 26 are embedded by any of various methods, suchas by an appropriate adhesive such as an epoxy adhesive, by cold or hotpressing, or by welding. Also located within an internal diameter ofeach of the tower-like protrusions 22 is a small cylindrical heat pipe28, also preferably secured in position with an appropriate adhesive.The heat pipes in the base and in the tower-like protrusions areelongate heat pipes, with the heat pipes in the tower-like protrusionsbeing substantially perpendicular to the heat pipes in the base.

Side, end and top views of the assembly of FIG. 1 may be seen in FIGS.2, 3 and 4, respectively. As shown in these Figures, the tower-likeprojections from the top surface of the base 20 are flanged cylindricalmembers having a cylindrical body 30 and a plurality of fin-likeprojections 32 thereon. The cylindrical members 30 provide support forthe fin-like members 32, yet allow sufficient space there between forconvection currents, either free or forced convection, to pass betweenthe cylindrical members, the fin-like projections 32 providing enhancedsurface area for maximizing the effectiveness of such convection.

In operation, the base 20 of the heat sink is cemented 36 to the topsurface of the packaged semiconductor device 34, typically but notnecessarily a ball grid array (BGA) package, as shown in FIG. 5.Typically, this will be done after functional testing of the systemwithin which heat sinks in accordance with the present invention areused. When so mounted, the horizontal heat pipes 26 will carry heatlongitudinally along the base 20 of the heat sink so as to provide amuch more uniform temperature distribution across the base of the heatsink. Similarly, the small cylindrical vertical heat pipes 28 will carryheat vertically from the base 20 of the heat sink upward through thetower-like projections 22 for conduction radially outward to thefin-like protrusion 32. Consequently, the heat is very effectivelydistributed across the area of the heat sink, and upward to the coolingfins for efficient convective cooling thereof.

In the preferred embodiment, the various members making up the assemblyother than the heat pipes themselves are preferably made of aluminum, arelatively light metal though quite a good heat conductor itself. Thebase 20 is preferably an extrusion having the openings 24 (FIG. 1) forthe heat pipes 26 directly formed therein. The tower-like protrusions 22are preferably manufactured from bar stock and secured in holes in thebase 20 provided for that purpose.

One of the advantages of the present invention is its modularity. Forinstance, the heat pipes are readily commercially available, andtherefore may be acquired on an as needed basis. The extrusions formingthe base of the heat sink may be fabricated either in only a single sizeor in very few sizes, depending upon the specific needs of varioussystems within which the present invention may be used, such as inmulti-chip modules and high power density and multi-power devices.Similarly, the tower-like projections 22 may be fabricated in bulk andused on an as needed basis. In that regard, note that while twelve suchprojections are shown in the embodiment of FIGS. 1 through 5, a lessernumber could be used for integrated circuits dissipating less power, anda lesser number would be used for smaller heat sinks for smallerintegrated circuits. Similarly, a greater number of tower-likeprojections 22 could be used on larger integrated circuit packages forfurther enhancement for the cooling thereof. Also, what ever number ofcavities for heat pipes are provided in the base member and in thetower-like projections, clearly a lesser number of heat pipes may infact be used if the full heat sinking potential of the device is notneeded.

As a first alternate embodiment, the cylindrical heat pipes 28 (seeFIG. 1) could be eliminated if desired. While doing so would reduce theheat sinking capability of the device, such an alternate embodimentwould be a lower cost heat sink providing adequate heat sinkingcapabilities in at least some applications. In such an alternateembodiment, the tower-like projections 22 may have the central holetherein left empty in the interest of commonalty of parts with thoseembodiments which utilize the heat pipes 28, or alternatively may havethe axial region thereof left solid aluminum for enhanced vertical heatconduction.

The embodiments described so far use tower-like projections that aresurfaces of revolution, and as such, are in general non-directional, inthat air flow may be directed over the heat sink in any horizontaldirection. Of course, in a typical system the direction of convectioncurrents, particularly forced convection currents, will be known bydesign so that the heat sinks in accordance with the present inventionmay be made direction dependent, provided care is taken to be sure thatthe same are appropriately aligned with the direction of air flow. Suchan embodiment is shown in FIG. 6. Here, as before, a base 40 having heatpipes 26 therein is provided. On the top surface of the base 40 in thisembodiment are a plurality of streamlined or approximately air foilshaped projections 42, each of which may have a small cylindrical memberheat pipe 28 therein. Projections 42 are disposed on the base 40 in rowsseparated by one or more air flow control walls 44, which of course alsoconduct heat upward from base 40 and provide an enhanced surface areafor removal of heat by convection.

In the embodiment of FIG. 6, the walls 44 may be extruded integral withbase 40, and of course the streamlined members 42 may themselves beextruded and sliced from the extruded bar, or die cast separately.Alternatively, if the openings for the heat pipes 26 extended throughthe bottom of the base 40, the entire part, excluding heat pipes, mightbe die castable using a simple two-piece mold, or cold forged using aone-piece die. In any event, the design remains modular, as one canadjust the number of heat pipes used based on the requirement of eachspecific application.

Now referring to FIG. 7, a still further embodiment of the presentinvention may be seen. This embodiment, like that of FIG. 6, is alsodirectional, in that forced convection should be in the x direction formaximum cooling effect and lesser resistance to air flow, or air/liquidcooling impingement, if used. Here, the base 50 containing heat pipes 26has a plurality of vertical projections 52 which also may be die cast orformed as extrusions and cut to length for mounting on the base 50. Aswith the prior embodiments, of course, the number of heat pipes used maybe varied in accordance with the requirements of any specificapplication.

The embodiment of FIG. 7 is intended for use in forced convectionsituations, though is also suitable for use for free convection cooling.In particular, since heated air rises, the fin-like protrusions 52 inFIG. 7 will encourage air flow horizontally into the finned region andthen allow the air heated by the fins to rise vertically, drawing inmore lateral cooling air. Also like the embodiment of FIG. 6, extendingthe opening for the horizontal heat pipes through the bottom surface ofthe base 50 would potentially allow this configuration to be die cast ina single piece like that described with respect to FIG. 6.

There have been described herein heat sinks of a modular characterwherein heat sinks of various sinking capabilities may be assembled fromcommon parts, dependent upon the particular requirements of the packagedintegrated circuits on which the heat sinks are to be used. One of theimportant enhancements of this invention is the ability to provideuniform and even heat flux (power density) distribution. This particularfunctionality reduces high junction temperatures in a semiconductor (orany power) device (ASICs, microprocessors, power and laser devices,etc.). This functionality will also enhance and reduce the coolingrequirements for these devices because of the heat flux per unit area(i.e., power density) reduction.

The volumetric and surface area heat flux distribution is determined bythe number and spacing of these heat pipes in both lateral and axialdirections, and the available cooling medium. In most cases, for lowpower devices, the application of this invention will be sufficient tocool devices by natural convection.

Thus while the present invention has been disclosed and described withrespect to certain preferred embodiments thereof, it will be understoodby those skilled in the art that various changes in form and detail maybe made therein without departing from the spirit and scope of theinvention.

What is claimed is:
 1. A heat sink for fastening to an integrated circuit comprising: a planar base member having an upper surface and an opposing lower mounting surface for fastening to an integrated circuit and receiving heat from the integrated circuit through the lower surface, the planar base member having a first heat pipe embedded therein having a length and width, the length exceeding the width, with the length of the first heat pipe being substantially parallel to the lower surface, the first heat pipe transferring heat across the planar base member to provide a more uniform temperature distribution across the base member; the planar base member having at least one protrusion projecting from the upper surface, the protrusion having a second heat pipe embedded therein having a length and width, with the length of the second heat pipe being substantially inclined with respect to the mounting surface, the entire second heat pipe being spaced further from the mounting surface than the entire first heat pipe, the first heat pipe and the second heat pipe having intersecting axes, the second heat pipe being thermally coupled to the first heat pipe and transferring heat within the protrusion.
 2. The heat sink of claim 1 further comprising a plurality of first and second elongate heat pipes, wherein the base member has the plurality of the first elongate heat pipes therein with the length of the first elongate heat pipes being substantially parallel to the mounting surface; the planar base member having a plurality of protrusions projecting therefrom, the protrusions each having one of the second elongate heat pipes therein with the length of each of the second elongate heat pipes being substantially perpendicular to the mounting surface.
 3. The heat sink of claim 2 wherein the protrusions are in the shape of surfaces of revolution.
 4. The heat sink of claim 2 wherein the protrusions are each unsymmetrical, providing a preferred direction of air flow for convection cooling.
 5. A heat sink comprising: a planar base member having an upper surface and an opposing lower surface to be thermally coupled to a planar surface of a heat source; a tower-like member having a first end, and an opposing second end, the tower-like member affixed to the upper surface of the planar base member at the first end, the tower-like member having a first axis connecting the first end and the second end that is substantially inclined to the planar surface; a first heat pipe having a first generally cylindrical surface with a second axis, the first heat pipe embedded in the tower-like member, the second axis generally parallel to the first axis; a second heat pipe having a second generally cylindrical surface with a third axis, the second heat pipe embedded in the planar base member, the third axis generally parallel to the planar surface and intersecting the second axis, the second heat pipe located between the planar surface and the first heat pipe, the second heat pipe thermally coupled to the planar base member and to the first heat pipe.
 6. The heat sink of claim 5 where in the tower-like member further comprises a cylindrical body and a plurality of fin-like projections affixed to the cylindrical body.
 7. The heat sink of claim 5 wherein the tower-like member is air foil shaped.
 8. The heat sink of claim 5 wherein the planar base member further comprises a first plurality of cavities that are selectively populated by a second plurality of tower-like members to provide a predetermined power dissipation.
 9. The heat sink of claim 5 further comprising a first plurality of tower-like members that are selectively populated by a second plurality of embedded heat pipes to provide a predetermined power dissipation.
 10. The heat sink of claim 5 wherein the heat source is an integrated circuit.
 11. The heat sink of claim 5 wherein the first axis is substantially perpendicular to the planar surface.
 12. An integrated circuit in a package with a substantially planar surface, the integrated circuit comprising a heat sink, the heat sink including: a planar base member having an upper surface and an opposing lower surface affixed and thermally coupled to the planar surface of the integrated circuit; a plurality of tower-like members each having a first end, and an opposing second end, each tower-like member affixed to the upper surface of the planar base member at the first end, each tower-like member having a first axis connecting the first end and the second end that is substantially inclined to the planar surface of the integrated circuit; a plurality of first heat pipes each having a first generally cylindrical surface with a second axis, each first heat pipe embedded in one of the tower-like members, the second axis generally parallel to the first axis; a second heat pipe having a second generally cylindrical surface with a third axis, the second heat pipe embedded in the planar base member, the third axis generally parallel to the planar surface and intersecting the second axes, the second heat pipe located between the planar surface and the plurality of first heat pipes, the second heat pipe thermally coupled to the planar base member and to the plurality of first heat pipes.
 13. The integrated circuit of claim 12 wherein each tower-like member further comprises a cylindrical body and a plurality of fin-like projections affixed to the cylindrical body.
 14. The integrated circuit of claim 12 wherein each tower-like member is air foil shaped.
 15. The integrated circuit of claim 12 wherein the planar base member further comprises a first plurality of cavities that are selectively populated by a second plurality of tower-like members to provide a predetermined power dissipation.
 16. The integrated circuit of claim 12 wherein a first plurality of tower-like members are selectively populated by a second plurality of embedded heat pipes to provide a predetermined power dissipation. 